330 MARINE INVERTEBRATES 
weight, began to sag over to one side. Such a process would of 
course tend to twist the digestive, tract and bring the excretory 
opening constantly farther forward on one side. At the same 
time, this process would necessarily interfere, by the weight of 
the overhanging viscera and the shell covering the mass, with the 
development of the branchiz lying upon that side and now crowded 
under the visceral mass. Thus, according to this theory, this 
process has continued until, in the modern gasteropod, the diges- 
tive tract has been bent upon itself and twisted from a straight 
course into a curve of almost 180°, bringing the excretory 
opening near or just over the head, where it empties into the 
mantle cavity; while the original right gill, by the same move- 
ment, has been brought to a position on the left side of the 
head, forward of the heart, the original left gill having been 
crowded out, eventually to atrophy and disappear. The same 
torsion is found in the nerve-cords; the heart, situated at about 
the pivotal position of this twisting process, has turned about 
upon itself, leaving an auricle in front of the ventricle, one 
auricle, like one of the gills, having been lost. Tite, mantle por- 
tion covering the visceral hump naturally continued to secrete its 
shell, though always in conformity with the change, tle result 
being the familiar spiral form of the usual gasteropod shell. 
This theory may not be satisfactory, but the asymmetry of 
gasteropods is a problem to be solved, and a more interesting 
line of biological investigation could not be found. 
Let us now take a good example of a gasteropod and locate its 
various organs; at the same time we may use the occasion to 
refer to more important modifications of these organs which will 
be encountered later in the various genera. The most available 
gasteropod on the east coast of the United States for this 
purpose is Fulgur, both on account of its large size and its 
abundance. Buccinuwm may be used if the student is north of 
Cape Cod and therefore unable to secure a good living specimen 
of Fulgur; the anatomical differences between the two are slight. 
Note the siphon protruding forward from a notch in the shell. 
This consists merely in an elongation of a fold of the mantle, 
which is held in a manner to constitute a tube, through which 
